Undersea Microbes Active But Living on the Slow Side (Forwarded)

Pennsylvania State University

Contacts:
A'ndrea Elyse Messer, (814) 865-9481
Vicki Fong, (814) 865-9481

Embargoed: February 20, 2006, 5 p.m. EST

Undersea Microbes Active But Living on the Slow Side

University Park, Pa. -- Deeply buried ocean sediments may house
populations of tiny organisms that have extremely low maintenance energy
needs and population turnover rates of anywhere from 200 to 2,000 years,
according to an international team of researchers.

"The microbial ecosystem in deeply buried marine sediments may comprise
a tenth of Earth's living biomass, but little is known about the
organisms, their physiologies, and their influence on surface
environments," says Jennifer F. Biddle, graduate student in
biochemistry, microbiology and molecular biology and member of the
NASA-sponsored Penn State Astrobiology Research Center.

The populations of interest are two groups of Archaea -- tiny
bacteria-like organisms that are often found in extreme environments
such as deep-sea hot vents, inside cows or termites or in deep
sediments. The samples were gathered during the National Science
Foundation-sponsored Ocean Drilling Program Leg 201 off the coast of Peru.

"The samples showed strikingly elevated concentrations of cells in
deeply buried sulfate-methane transition zones," says Dr. Christopher H.
House, assistant professor of geosciences, Penn State. "Sulfate methane
transition zones are areas where both methane and sulfate diffuse and
both compounds are used by local denizens."

The researchers looked for 16S rRNA in the sediment samples and found
the transition zones dominated by two groups -- Marine Benthic Group B
and Miscellaneous Crenarchaeotal Group. rRNA is found in a cell's
ribosome and is part of the protein manufacturing mechanism of a cell.
The presence of a specific sequence of 16S rRNA distinguishes the types
of Archaea and the analysis also identifies Archaea that are active,
excluding inactive cells and fossils.

"Other researchers have found DNA analysis of sediments from some sites
to indicate that the majority of organisms were Bacteria and not
Archaea," says House. "We used methods that identify only active cells
and found Archaea."

Another method of identifying the active populations -- both in size and
type -- looked at intact polar lipids, an indication of live rather than
fossil cells.

"These tests and others indicate that there is a sizeable and active
archaeal community," says House.

Besides simply knowing that populations of Archaea exist in the deep
sediment layers at the sulfate-methane transition zones, the researchers
looked at the energy sources for these microbes. Many organisms living
in environments with methane use the methane for energy and use the
methane's carbon to grow, repair and reproduce. Looking at the carbon
isotopes the researchers found that few, if any, of these Archaea used
methane as a carbon source. They also found that conversion of carbon
dioxide to methane was not fueling these Archaea.

"Because the carbon isotopes from the Archaea match the total organic
carbon found in the sediment in general, it suggests that the bulk
archaeal community uses organic compounds derived from fossil organic
matter," says House.

The researchers suggest in this week’s issue of the Proceedings of the
National Academy of Sciences online, that degradation of organic matter
in the sediment, especially the formation of small molecules like
acetate and formate, are the likely sources of carbon.

"Real maintenance energies in subsurface environments must be much lower
than what has been experimentally determined in laboratory cultures,"
says Biddle. "If conventional maintenance energies are used, only about
2 percent maximum of the population could survive. However, cellular
maintenance energies are expected to be significantly lower when cells
divide at extremely low rates."

In fact, the researchers estimate that these Archaea may completely turn
over population as frequently as every 70 years, or as infrequently as
2,150 years. They also suggest that the sulfate-methane transition zone
is a much better environment than other areas in the sediment and that
turnover rates are even lower away from the transition zone.

This is because the Archaea in the transition zone, while not using the
carbon from methane oxidation, are still getting some energy from
breaking down the methane molecules, energy that is not available in
other portions of the sediment.

"These Archaea subsist on the sedimentary organic carbon available and
the energy from breaking down methane until they accumulate enough
resources to divide," says House. "Surprisingly they require much less
energy to maintain and take much longer than expected until they can
divide."

This international research team was lead by House, Kai-Uwe Hinrichs
from the University of Bremen and Woods Hole Oceanographic Institution,
and Andreas Teske from the University of North Carolina. The team
included graduate students Biddle, Julius S. Lipp, Mark Lever and Karen
Lloyd.

The National Science Foundation, NASA Astrobiology Institute, Deutcsche
Forschungsgemeinschaft and the U.S. Department of Energy supported this
work.

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EDITORS: Dr. House is at 814-865-8802. Dr. Hinrichs is at +49-421-218-65700.